A roller charged with lithographic ink (No. 2, mixed with middle varnish, is the best), is then passed over it, backwards and forwards, several times, so that more ink may adhere to the lines. Coarser resin is now sifted on, and it is heated more strongly. This causes the ink to melt, and cover the resin fringe, and thus form a secure acid-proof varnish. To a careful observer, the lines will appear thicker after every heating. The strength of the acid is now increased to 5 per cent., and the plate is bitten about *015 in.; when dry, it is again resined, heated, cooled, gummed, inked, resined and heated just as before, each time the processes being carried further than the last. Ordinary plates - such as the illustrations to the Figaro - have to be bitten about 8 to 12 times, while very fine ones, such as transfers from steel engravings, have often to be etched as many as 20 times - of course proportionately less for each time. These instructions will enable any one of ordinary manipulative ability to engrave zinc plates very fairly, but it requires something more than mere formulae and advice to finish off the fine tint and crayon drawings so beautifully and delicately as they are done by Gillot, Zim merman, and other clever French engravers.
Years of practice are wanted, and each individual has his own particular pet mode of working which suits him best, and which he considers as superior to all others, and jealously retains as a trade secret.
(26) Phototypy is a sort of lithography in which the stone is replaced by a hygroscopic layer of gelatine impressed with an image by the action of light passing through a photographic negative. Now, if we could cut down a lithographic stone both in its surface dimensions and its height, to make it like a wood block, we should be able to insert it in the text, and take an impression from it simultaneously with that from the type. The difficulties in the way of doing this would be, firstly, the necessity of wetting the stone previous to each impression; and, secondly, the expense of cutting down lithographic stones, which would entirely lose their value in the process. But what we are on this account prevented from effecting with natural lithographic stones can be managed with an artificial one, provided that the latter possesses a hygroscopic surface from which, after being saturated with water, numerous impressions can be taken without its being necessary to wet it afresh. It became, therefore, necessary to make photo-printing blocks of the requisite size and height to be set up in the "forme" with ordinary type, and possessing so great a hygroscopic quality that the moistening requisite to produce an impression should only be an accidental operation, and not one that is indispensable before each pull.
The ordinary process of photo-typy was scarcely adapted for this purpose without modification. The plates in this process are made of metal or glass, or even lithographic stone, always larger than the image of which it is required to obtain an impression, and it would be impossible in every case to cut these plates to the size of the printing block. Vidal adopts another method for arriving at the same result as that produced by ordinary phototypy. He prepares the artificial lithographic stone and the hygroscopic support separately, and then attaches the one firmly to the other. The image is obtained as in the ordinary carbon process; an impression on carbon tissue is developed on a roughened glass plate coated with some fatty substance. When, by means of hot water, the picture is divested of all the gelatine not acted on by light, it ought to appear with all its half-tones like a good carbon print which is ready to be transferred to its definite support. This is then enclosed in a frame of thick cardboard, bevelled outwards on the inside, and coated entirely with paraffin or wax; the frame is then filled with the following composition, which is poured into it and over the picture: - 308 gr. gelatine, 30 3 gr. gum arabic, 616 gr. glycerine, 3 1/2 fl. oz. water, 17 fl. oz. ammonia, 8 gr. alum, 185 gr. salicylic acid, 154 gr. barium sulphate.
The salicylic acid is added as an antiseptic, and the sulphate of barium gives to the layer of gelatine an opalescent appearance. The whole layer should be so deep as to have, after drying in the chloride of calcium box, a thickness of about 0.2 in. When the desiccation is complete, the layer above the glass plate is turned out, and will be found to have the image transferred to it. We have now, therefore, a plate of gelatine bearing on it the picture of the exact dimensions required, and bevelled downwards from the edges, which latter will, therefore, not take any ink. This plate must then be mounted on a sheet of copper or zinc, which is raised on a wooden support until the height of the image is the same as that of the type with which it is to be printed. The gelatine plate is next saturated with moisture by immersion for 1/4 hour in a bath composed of If oz. glycerine, 1.69 fl. oz. water, 31 gr. alum, and the image will appear on its surface in considerable relief, so as to render it particularly well adapted for printing from. The separation of the black parts of the picture from the white parts of the hygroscopic gelatine is very perfect, so that no smudging, such as so often occurs with printing blocks on which the shadows are modelled by fine lines close together, need be feared.
The mixture of which the formula is above given is of so hygroscopic a character that repeated wetting is rarely necessary. It must be effected with a sponge dipped in a mixture half water and half glycerine, after having removed from the plate all trace of ink; but the latter should never be severely washed. In this way is obtained a carbon print, but with a light-coloured pigment, so that the degree of inking can be readily determined. Light-coloured earths in the form of impalpable powders, with a gelatine chosen for its resistent properties, make very good tissue. The print should not be treated with alum before putting on the layer of hygroscopic gelatine, otherwise it will not transfer easily. On the contrary, it is better to wash it with water containing a little ammonia, which will facilitate the penetration; the mixture already contains some ammonia, and the transfer of the image to the plate of glycerine and gelatine is thus rendered completely effectual. The alum contained in the first liquid used for moistening increases the hardness of the image, and prevents it from swelling too much. (Brit. Jl. Photography.)